The present invention relates to a copier of the type automatically transporting an original document to and stopping it at a predetermined position on a glass platen by an automatic document feeder (ADF) and then moving optics for exposure in the form of a scanner to illuminate the document. More particularly, the present invention is concerned with a copier of the type described which is operable at an extremely high copying speed.
Generally, a copier having optics in the form of a scanner and an ADF is provided with a glass platen the size of which is large enough to allow at least a document of size or format A3 to be laid laterally long with respect to an intended direction of document feed. This type of copier is in many cases constructed such that documents and paper sheets are fed from the right to the left as viewed from the front of the copier and, in the event of exposure, the scanner is transported from the left to the right, for the purpose of facilitating manual operations. For this reason, it is a common practice with such a copier to stop and position a document on the glass platen by using the left edge of the glass platen for a reference. The scanner is usually located at a home position which is slightly leftward of the left edge of the glass platen, i.e., reference position and, at the instant of exposure, it is moved to the right away from the home position to scan the document which is held in the reference position on the glass platen.
A problem with the prior art copier with an ADF described above is that a document has to be fed rightward until its leading edge reachs the reference position defined at the left end of the glass platen, with no regard to the size of the document. More specifically, the same period of time is needed for a document of comparatively small size (e.g. format A4) to be located at the reference position as needed for a document of comparatively largest size usable with the copier. Hence, the copying rate is far lower when different documents are sequentially fed in a continuous copy mode than when the same document is reproduced in a repeat copy mode, whatever the size of the documents may be.
To eliminate the above problem, an arrangement may be made such that the size of documents to be automatically fed from a document tray is sensed beforehand and the stop position (sheet-through mode) or the home position (optics transport mode) of the optics is changed in matching relation to the sensed document size, as disclosed in U.S. patent application Ser. No. 07/199,869 entitled "AUTOMATIC DOCUMENT FEEDER" by way of example. Although this kind of implementation is successful in increasing the copying rate to a substantial extent by changing the position of the optics, a document is still fed to a predetermined reference position on the glass platen with no regard to its size and, therefore, the copying rate cannot be increased beyond a certain limit.